In recent years, together with prevalence of solar-light power generation systems as well as a start of an excess-power wholesale purchase system for solar-light power generation systems, there have been growing needs, also in ordinary homes, for knowing generated power and sold power (excess power) of a solar-light power generation system, purchased power from commercial power supplies, power consumption within homes, and the like.
Conventionally, there have been proposed techniques for detecting which is a currently running state, one state that excess power of a private power generation system, such as a solar-light power generation system, is being supplied to a commercial power supply side as is regarded as an electric power selling state (hereinafter, referred to as power selling state) or the other state that electric power is being supplied from a commercial power supply as is regarded as an electric power purchasing state (hereinafter, referred to as power purchasing state) (see, e.g., Patent Literatures 1 to 3). The inventions of Patent Literatures 1 to 3 include the steps of measuring voltage and current of an electric power line on the commercial power supply side, calculating an electric power from the measured voltage and current, detecting a direction of power flow based on a sign (positive or negative) of the calculated power, and making a decision as to which of the power purchasing state and the power selling state is the currently running state.